A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. These target portions are commonly referred to as “fields”.
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, the accuracy of alignment of two layers in a device. Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a diffraction “spectrum” from which a property of interest of the target can be determined.
Examples of known scatterometers include angle-resolved scatterometers of the type described in US2006033921 A1 and US2010201963 A1. The targets used by such scatterometers are relatively large, e.g., 40 μm by 40 μm, gratings and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). In addition to measurement of feature shapes by reconstruction, diffraction based overlay can be measured using such apparatus, as described in published patent application US2006066855 A1. Diffraction-based overlay metrology using dark-field imaging of the diffraction orders enables overlay measurements on smaller targets. Examples of dark field imaging metrology can be found in international patent applications US2014192338 and US2011069292 A1 which documents are hereby incorporated by reference in their entirety. Further developments of the technique have been described in published patent publications US20110027704 A, US20110043791 A, US2011102753 A1, US20120044470 A, US20120123581 A, US20130258310 A US20130271740 A and US2016091422 A1. These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Multiple gratings can be measured in one image, using a composite grating target. The contents of all these applications are also incorporated herein by reference.
An overlay measurement is typically obtained by measuring asymmetry of two overlay gratings, each having a different programmed (deliberate) offset or “bias”. Although, the overlay measurements are fast and computationally very simple (once calibrated), they rely on an assumption that overlay (i.e., overlay error and deliberate bias) is the only cause of asymmetry in the target. Any other asymmetry in the target perturbs the overlay measurement, giving an inaccurate overlay measurement. Asymmetry in the lower or bottom periodic structure of a target is a common form of structural asymmetry. It may originate for example in substrate processing steps such as chemical-mechanical polishing (CMP), performed after the bottom periodic structure was originally formed.
When using two or more biased grating structures to obtain an overlay measurement, the known method further assumes that the two gratings are identical, apart from the bias. It has been discovered that, in addition to or alternatively to structural asymmetry in a target, a difference between adjacent periodic structures of a target or between adjacent targets may be a factor that adversely affects the accuracy of measurement, such as overlay measurement. This difference may be referred to as “stack difference”, and encompasses any unintentional difference in physical configurations between periodic structures or targets. Stack difference imbalance includes, but is not limited to, a thickness difference in one or more layers of the stack between the adjacent periodic structures or targets, a refractive index difference between the adjacent periodic structures or targets, a difference in material between the adjacent periodic structures or targets, a difference in the grating CD or height of the structures of adjacent periodic structures or targets, etc. Like structural asymmetry, the stack difference may be introduced by processing steps, such as CMP, layer deposition, etc. in the patterning process.
When such structures are measured for the purposes of an overlay measurement, the stack difference may influence the measurement signals. Where asymmetry is measured through a difference between diffraction signal of opposite diffraction orders, the stack difference may cause for example a difference in the average of the diffraction signals as well. The influence of the stack difference that is represented in the measurement signals may be referred to as “grating imbalance”.